This invention relates to a method of cooling an immersed induction coil such as an ionising coil used in a vacuum processing chamber.
Many problems can arise if the temperature of ionising coils are not properly controlled and this is particularly true where the coils are inside the vacuum processing chamber, because in that case the adhesion of deposited material on the coil may be incomplete giving rise to damaging particles within the processing chamber. Traditionally such coils have been cooled by flowing a liquid coolant constantly along a cooling passage extending through the coil. If water is used then the cooling is if anything too efficient and the coil is kept well below the ambient temperature of the process chamber, resulting in high levels of added particles to processed wafers. Conversely, if air is used then the cooling is in principle about right, but with a coil of any length and this includes even single term coils, a substantial thermal gradient can build up along the length of the coil causing delamination of deposited material.
The present invention consists in a method of cooling an induction coil having a cooling passage extending through it, including flowing coolant through the cooling passage in the first direction and then flowing coolant through the passage in the opposite direction.
The direction may be switched periodically, with for example, the switching period being reduced the longer the process runs and additionally or alternatively it may be controlled in response to detected coil temperature. Most conveniently the method may include monitoring the down stream temperature of the coolant and/or the coil and reversing the direction when that temperature exceeds a predetermined level.
It is particularly preferred that the coolant is air or some other suitable gas.
Although the invention has been defined above it is to be understood it includes any inventive combination of the features set out above or in the following description. The invention also includes apparatus for performing the method.